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New Evidence Standards Under California Senate Bill 652

Attorney: Michelle Watkins, Joshua C. Anaya | Published 8.24.23

California Governor Gavin Newsom recently approved California Senate Bill 652, affecting a defendant’s ability to introduce expert opinion evidence on alternative medical causation at trial. Specifically, the law abrogates the Second District Court of Appeal’s recent ruling in Kline v. Zimmer (2022) 79 Cal.App.5th 123 and adds a new section to Evidence Code, section 801.1, which takes effect on January 1, 2024, and creates additional admissibility requirements for defendant expert opinions formerly reserved for plaintiff expert opinions. These changes will impact defense trial and discovery strategies for attacking medical causation through retained medical experts.


In Kline, the plaintiff underwent unsuccessful surgery to implant an artificial hip joint manufactured and designed by the defendant. Plaintiff sued the defendant for product defects and alleged that the implant’s faulty design resulted in ongoing hip pain, necessitating at least two additional surgeries. At trial, the defendant attempted to introduce causation opinions and other evidence to challenge the medical causation opinions of the plaintiff’s expert. The trial court excluded the defendant’s evidence because it was not proffered within a reasonable degree of medical probability, and the jury returned a verdict for the plaintiff. The Court of Appeal reversed the judgment and remanded for retrial. The Kline court reasoned that a plaintiff in a personal injury action must prove that a defendant’s wrongful conduct caused the injury alleged and that the opinions of the plaintiff’s experts in that regard must be expressed within a reasonable degree of medical probability. However, defense experts’ alternative causation theories need not meet the same standard of medical probability because the defendant carries no such burden of proof at trial. (Kline v. Zimmer, supra, at p. 133-134.) [Emphasis added.]


Despite various entreaties, the California Supreme Court declined to review or overrule the Kline decision. Subsequently, California Senate Bill 652 was introduced and sponsored by The Consumer Attorneys of California to ensure that all expert witness opinion testimony offered by the plaintiff, or the defendant is based on a reasonable degree of probability in an expert’s particular medical field of expertise in the interests of fairness. The newly proposed language is as follows:


Evidence Code section 801.1.


(a) Where the party bearing the burden of proof proffers expert testimony regarding medical causation and where that party’s expert is required as a condition of testifying to opine that causation exists to a reasonable medical probability, the party not bearing the burden of proof may offer a contrary expert only if its expert is able to opine that the proffered alternative cause or causes each exists to a reasonable medical probability, except as provided in subdivision (b).

(b) Subdivision (a) does not preclude a witness testifying as an expert from testifying that a matter cannot meet a reasonable degree of probability in the applicable field, and providing the basis for that opinion.


By way of the newly adopted language, a defendant and her experts now carry the same burden as the plaintiff and his experts, contrary to the reasoning in Kline. Though advocates of the new law believe it promotes a more even playing field around the parameters of expert testimony in California courts, in practice, the statutory shift may act to create new hurdles for defendants. In the context of personal injury and product defect cases, the new language effectively limits defense attacks on a plaintiff’s prima facie case for medical causation if that attack is based on evidence with less than reasonable medical probability. In these situations, alternative causation is not an affirmative defense for which the defendant carries the burden of proof but an element of the plaintiff’s burden of proof.


Any restriction on introducing expert opinion evidence affects trial preparation, expert discovery, and settlement strategies. SB 652 and its changes to the Evidence Code will impact not only cases headed to trial in 2024 but also cases set for trial between the date of this article and before the law’s effective date of January 1, 2024.


For more information on the upcoming changes to the law, as well as how it can affect the defense of pending and future trials, please contact Michelle Watkins or Joshua Anaya.